In induction heating devices, an alternating magnetic field is generated by means of an induction heating coil, the said alternating magnetic field inducing eddy currents in a cooking vessel which is to be heated and has a base which is composed of ferromagnetic material, and creating remagnetization losses, as a result of which the cooking vessel is heated.
The induction heating coil is a constituent part of a resonant circuit which comprises the induction heating coil and one or more capacitors. The induction heating coil is usually designed as a flat, helically wound coil with associated ferrite cores and is arranged, for example, beneath a glass-ceramic surface of an induction hob. In this case, the induction heating coil forms an inductive and a resistive part of the resonant circuit in conjunction with the cookware which is to be heated.
In order to drive or excite the resonant circuit, a low-frequency mains AC voltage with a mains frequency of, for example, 50 Hz or 60 Hz is first rectified and then converted into an excitation or drive signal with a higher frequency by means of semiconductor switches. The excitation signal or the drive voltage is usually a square-wave voltage with a frequency in a range of from 20 kHz to 50 kHz. A circuit for generating the excitation signal is also called a (frequency) converter.
Various methods are known for setting a heating power supply to the cooking vessel as a function of a set heating power setpoint value.
In a first method, a frequency of the excitation signal or of the square-wave voltage is varied as a function of the heating power which is to be output or to be supplied or of the desired power conversion. This method for setting the heating power output makes use of the fact that a maximum heating power is output when the resonant circuit is excited at its resonant frequency. The greater the difference between the frequency of the excitation signal and the resonant frequency of the resonant circuit, the lower is the heating power which is output.
However, if the induction heating device has a plurality of resonant circuits, for example if the induction heating device forms an induction hob with different induction cooking points, and different heating powers are set for the resonant circuits, beats which can lead to interfering noise can be produced by superimposition of the different frequencies of the excitation signals.
One method for setting the heating power which avoids interference noise resulting from such beats is pulse-width modulation of the excitation signal at a constant exciter frequency at which an effective value of a heating power is set by means of varying the pulse width of the excitation signal. However, high switch-on and switch-off currents are produced in the semiconductor switches in the case of effective value control of this kind by varying the pulse width at a constant exciter frequency, as a result of which a broadband and high-energy interference spectrum is produced.
It is often desirable to determine a temperature of a cooking vessel base which is inductively heated in this way, in order to be able to generate, for example, specific heating time profiles, to determine a boiling point and/or to enable automatic cooking functions.
DE 10 2009 047 185 A1, which corresponds to pending U.S. Patent Application No. 2011/0120989, discloses a method and an induction heating device in which temperature-dependent ferromagnetic properties of the cooking vessel base are measured with a high resolution and are evaluated for determining the temperature of the cooking vessel base.